The present invention relates to a heat collector for a solar heat utilization system.
A heat collector of this type generally has a heat collecting pipe and a reflecting plate which reflects solar light to focus the light on the pipe. The pipe is heated by the direct solar light and the light reflected by the reflecting plate, and the heat is transferred to a heating medium which flows in the pipe. The pipe is arranged in a vacuum glass cylinder, thereby preventing heat from radiating from the pipe. Since the light reflected from the reflecting plate is focused on the tubular wall of the pipe at the side of the plate, the temperature at that wall becomes higher than the wall at the side facing the sun, and a temperature difference of a predetermined rate is produced between the tubular walls at the side of the plate and at the side facing the sun. Expansion difference is produced by this temperature difference between the tubular walls of the pipe, with the result that the pipe bends. When the pipe is thus bent, the light reflected from the reflecting plate is not focused on the pipe, resulting in a decrease in the heat collecting efficiency of the heat collector. When the pipe is greatly bent, the pipe is forced against the inner surface of the glass cylinder, causing the cylinder to break.